A Day on Venus Is Longer Than a Year

If you lived on Venus, you would complete an entire orbit around the Sun before you finished a single day. That is not a metaphor or an exaggeration — it is a straightforward consequence of how slowly Venus spins.

A year on Venus — the time it takes to orbit the Sun — lasts about 225 Earth days. That is already shorter than many people expect, given that Venus is closer to the Sun than Earth is. But Venus rotates on its axis extraordinarily slowly. One full rotation takes approximately 243 Earth days. This means a Venusian day is about 18 Earth days longer than a Venusian year.

To make things even stranger, Venus spins in the opposite direction to most planets. On Earth, the Sun rises in the east and sets in the west. On Venus, it would be the other way around — the Sun rises in the west and sets in the east. If you could stand on the surface and watch the sky, the Sun would crawl slowly westward across the sky at an almost imperceptible pace.

Scientists are not entirely certain, but the leading explanation involves a massive collision early in the solar system's history. A large object may have struck the young Venus with enough force to dramatically slow its rotation and flip its spin direction. Similar giant impacts are thought to have shaped other unusual features of planets — Earth's own Moon is believed to have formed from just such a collision.

The slow rotation is far from the only strange thing about Venus. Despite being farther from the Sun than Mercury, Venus is actually the hottest planet in the solar system. Its surface temperature averages around 465 degrees Celsius — hot enough to melt lead. The culprit is a runaway greenhouse effect. Venus has a thick atmosphere of carbon dioxide that traps heat so effectively that temperatures remain scorching both day and night.

The atmospheric pressure on the surface of Venus is about 92 times greater than on Earth — equivalent to being nearly a kilometre underwater. Any spacecraft that has landed there has been crushed or melted within a matter of hours. The longest any lander has survived on the Venusian surface is about 127 minutes, achieved by the Soviet Venera 13 probe in 1982.

Venus is often called Earth's twin because the two planets are nearly identical in size and mass. Yet they could hardly be more different in practice. Earth teems with life; Venus is a crushing, superheated wasteland. The contrast stands as a reminder of how finely balanced the conditions for life must be — and how easily a planet can go in a very different direction.